1. Field of the Invention
The present invention relates to a patterned structure having minute slits, openings or other voids whose aspect ratio (ratio of the diameter or width to the thickness, i.e., diameter/thickness or width/thickness) is smaller than 1, and a method of producing such a minutely patterned structure.
2. Discussion of the Prior Art
Patterned structures having extremely small slits, openings or other voids, particularly, such patterned structures made of a metallic material, have been used as electrical lead frames for LSI packages, and as various masking members, for example. Techniques for fabricating a double-layer or bi-plane lead frame or similar product from two members, with improved density of the wiring formed thereon, are known according to various publications such as laid-open Publications 58-90752, 60-92646, 60-103652, 60-107848, 60-206144, 60-227457 and 62-81739 of unexamined Japanese patent applications. Generally, these techniques of fabrication utilize press-forming or etching methods. These methods are difficult to be practiced for forming slits, openings or other voids which have a small aspect ratio. Conventionally, therefore, the voids having a small aspect ratio are formed by ion-milling or laser-cutting methods.
Another technique for producing a patterned structure is disclosed in laid-open Publication 51-40869, wherein a plurality of thin sheets having the same configurations are superposed on each other to form a planar patterned assembly having a desired thickness.
However, such a planar patterned structure as indicated above suffers from extreme difficulty in precisely positioning the two or more sheets relative to each other, and therefore cannot provide a sufficiently high degree of dimensional or configurational accuracy of the formed pattern. Further, the thickness of the patterned structure is determined by the total thickness of the constituent sheets that are superposed on each other. Where the patterned structure has a portion or portions whose thickness should be accurately controlled, the desired thickness accuracy cannot be easily obtained. In the case of an electrical lead frame, the thickness or volume of resin, ceramic or other sealing materials increases with the thickness of the structure, i.e., total thickness of the constituent sheets. In this case, the heat dissipation characteristic of the lead frame is accordingly deteriorated.
On the other hand, the conventional ion-milling and laser-cutting methods do not permit sufficient efficiency in forming the voids, and may cause damage to the portions of the workpieces which should be kept intact. Further, these methods require extremely expensive equipment. If the workpiece sheet is reduced to reduce the width of the slits or the diameter of the openings, the strength and the uniformity of thickness of the prepared patterned structure tend to be reduced, and the patterned structure cannot be easily processed for practical application. Thus, the ion-milling and laser-cutting methods suffer from drawbacks in terms of the process to form the voids in the structure, and the physical properties and the dimensional and configurational accuracy of the structure.
In the alternative known method which uses two or more sheets having the same configuration to prepare a patterned structure, it is considerably difficult to establish precise alignment of the sheets relative to each other for accurate dimensioning of the pattern. Further, since the thickness of the patterned structure is determined by the total thickness of the sheets superposed on each other, it is difficult to precisely control the thickness of the desired local portions of the structure.